This invention relates to thermoplastic sheet of plasticized polyvinyl butyral (PVB) for use in safety glass and more particularly to a surface pattern in such sheet which reduces its tendency to stick to a like sheet.
Plasticized PVB sheet is used with optically transparent glass in laminated safety panels in vehicle and building windows and the like. It is known to roughen the sheet to create channels through which air exhausts from the interface of the sheet with the glass layer during the initial stage of preparing the safety glass laminate. Unremoved air appears as cloudy patches and detracts from the appearance of the finished laminate. U.S. Pat. No. 5,455,103 to Hoagland et al discloses a form of regular roughness pattern for surfaces of such sheet which optimizes air removal while eliminating an undesirable appearance problem referred to in that patent as moire patterns.
Plasticizer is necessarily present in rather significant amount in such thermoplastic PVB sheet to reduce stiffness and render it capable of absorbing impact forces when the finished glass laminate is forcibly struck. Unfortunately this makes the sheet incapable of self support at room temperature and rather sticky so that abutting layers tend to stick to each other unless stored before use at low temperature (e.g. about 10-15.degree. C.).
To improve efficiency and reduce manufacturing cost, various steps in preparing glass laminates containing plasticized PVB sheet as the shock absorbing layer are increasingly being automated. These steps include unwinding sheet from a supply roll, shaping the sheet by stretching to the approximate curved peripheral configuration of the glass laminate when, for example, to be used as a vehicle windshield and cutting the shaped sheet into blanks which are collected and stacked upstream of the glass laminating process. In the initial laminating step an automated robot is sometimes used to successively peel individual sheets from the top of the stack and transfer them to the next station for combination with glass. If a robot cannot separate the topmost blank from the next subjacent one because of sticking, an alarm rings to bring an operator for manual assistance. This adds cost and may create defects and off grade sheet because of rough handling necessary to separate the sticky abutting blanks.
The problem of sticking abutting layers of PVB sheet can also exist when the finished laminate is for architectural applications. In these applications windows are typically larger than in vehicles and the greater surface area of abutting layers aggravates the sticking problem. It may even be impossible to unwind laps of the sheet from its bulk supply roll, which condition is called roll blocking. This is minimized by storing rolls at reduce ambient temperature, but this requires refrigerated air in the storage area.
A need continues for plasticized PVB sheet with reduced tendency to stick to an abutting layer in a stack or lap of a roll while simultaneously deairing well in forming the glass laminate, even when initially shaped by stretching to conform to the laminate periphery.